My family's stereo system had been being unreliable for a while, so I decided to build a new one.
I had an Arduino (a small microcontroller board built around an ATMega 328p chip) on hand that I had previously casually messed with, so I decided to try my hand at building a replacement amplifier and stereo using the Arduino as the central controller. The amplifier itself is actually a kit, but I hacked its volume control and input/output to be controlled digitally by the Arduino. This allows for the user to individually set the volume of each speaker. The radio used is an Si4735 digital radio tuner talking digitally to the Arduino. Beyond the radio, the system also has 3 selectable auxillary inputs for the CD, Cassette, and Bluetooth receiver. Finally, it has 2 selectable stereo outputs so the same stereo can drive either the living room or basement speakers.